The present invention relates to a hydraulically operated mine prop, in which the end wall of the inner prop member bordering the fluid filled space of the outer prop member is provided with an overpressure safety valve of large flow-through cross-section and comprising a valve seat in communication with the fluid filled space of the outer prop member and a valve member axially movable between a closed position engaging the valve seat and an open position in which the fluid filled space of the outer prop member communicates through the valve seat with channels leading to the outside of the prop. The valve member is normally held in the closed position by a gas pillow under high pressure pressing the valve member against the valve seat.
In hydraulically operated mine props, an uncontrolled pressure build-up may occur in the fluid filled chamber of the prop in the event of a certain excessive loading of the prop, for instance during a rock burst or a sudden collapse of the mine roof supported by the prop. Such a pressure build-up may be caused by the relatively small cross-sectional flow-through area of the overpressure valve which is usually provided in order to protect the prop and by the inertia of the movable valve members which are unable to immediately reduce the pressure peaks occurring during rock bursts.
In order to avoid these disadvantages, special overpressure valves have been used which are provided with large flow-through cross-sections. However, such valves necessarily require high closing forces, which are further increased because such valves must generally be adjusted for closing forces which are basically higher than the closing forces acting on normal working valves. In principle, these high closing forces can be produced by mechanical springs. However, such springs require a large mounting space, which is usually not available in mine props of the type in question. Such mechanical spring elements have, therefore, been replaced with cushions of highly compressed macromolecular gases.
Gas cushions of this type have, however, a substantial disadvantage. Notwithstanding the fact that macromolecular gases are being used, these gases still have the tendency to diffuse gradually through the material of the seals, which seal the pressure chamber enclosing the gas cushion. Such a gas loss causes the compression of the gas cushion to decrease gradually. The function of the gas cushion thus ceases, with the result that the prop is no longer protected in the event of a sudden excessive loading. In order to meet the high safety requirements in underground mining, the over-pressure valves must, therefore, be taken out after a certain service life from the mine props and replaced by new valves. The therewith connected expenditure in time, personnel and material, is evidently rather high.